Labour has "run out of excuses" to oppose an early election, Boris Johnson has said, as MPs vote on whether to back his call for a December poll.
The PM said "nobody relished" going to the polls weeks before Christmas but this Parliament had "run its course" and was "incapable" of settling Brexit.
The PM has formally accepted the EU's offer of a Brexit extension until 31 January 2020 agreed earlier on Monday.
In a letter to EU officials, he said it was an "unwanted prolongation".
Urging the EU to rule out any further extension, he said there was time to ratify his Brexit deal but he feared the current Parliament would never do so "as long as it has the option of further delay".
The PM's acceptance means that the UK will not leave the EU on Thursday - despite a "do or die" promise he repeatedly made during this summer's Tory leadership campaign and since taking office in July.
EU Council President Donald Tusk said what was being offered was a "flextension" - meaning the UK could leave before the deadline if a deal was approved by Parliament.
It comes as MPs prepare to vote on the PM's proposals for an early general election on 12 December. The SNP and Lib Dems have also proposed an election - on 9 December. The vote is expected after 1900 GMT.
'Run and hide'
Speaking in the Commons, Mr Johnson said Labour was the only main opposition party resisting an early election, telling Mr Corbyn that he "can run but he cannot hide" from the electorate.
In response, Mr Corbyn said he backed an election but only after certain conditions were met - including legal confirmation of the extension and reassurances that students wouldn't be "disenfranchised" by the mid-December date because they had left for the Christmas holidays.
"The reason I am so cautious is that I do not trust the prime minister," he said. "Today he wants an election and his bill - not with our endorsement."
A No 10 source said the government would introduce a bill "almost identical" to the Lib Dem/SNP option on Tuesday if Labour voted their plan down later, and "we will have a pre-Christmas election anyway".
The SNP's Westminster leader Ian Blackford said it would only support a poll "on its terms" and suggested this could depend on 16 and 17-year olds and EU nationals being given the vote.
The UK was due to leave the EU on Thursday, but Mr Johnson was required to request an extension after Parliament failed to agree a Brexit deal.
The prime minister had repeatedly said the UK would leave on 31 October deadline with or without a deal, but the law - known as the Benn Act - requires him to accept the EU's extension offer.
The Lib Dem/SNP plan does not include a new timetable for his legislation - the Withdrawal Agreement Bill.
They want the 9 December because it would not leave enough time for the bill to become law before Parliament is dissolved - which must happen a minimum of 25 working days before an election.
The BBC's political editor, Laura Kuenssberg, said she understands the government has offered to fix the election date on 12 December, but a Lib Dem source told her they were holding firm to their date, adding: "If we are doing this, we are not doing it on the government's terms."
Understand government has offered the lib dems a bill they'd introduce tomorrow that would fix General Election on 12th, but lib dems holding firm to their date of the 9th at the moment - lib dem source says ‘If we are doing this, we are not doing it on the government’s terms’— Laura Kuenssberg (@bbclaurak) October 28, 2019
Labour MPs are expected to abstain in the Commons vote on a 12 December election.
It comes as government figures showed a surge in voter registrations, with nearly two million registering in the past eight weeks.
Over half of the applications - 58% - were from voters aged 34 or under, compared to just 7% for those over 65. The swell coincided with Mr Johnson's first proposal, in early September, for a snap election.
The EU has finally announced its informal approval of a new Brexit extension - but what an excruciatingly long and confusing political dance to get there. And the dance is not over yet.
To become a formal offer, the Brexit extension still needs to be accepted by UK PM Boris Johnson. This is EU law and an unavoidable part of the procedure.
But how uncomfortable for the prime minister who sought to distance himself as much as possible from the extension, previously promising that he would rather die in a ditch than request one.
The EU is also attaching some extra wording to the extension - including a reminder for the UK that, until it leaves, it remains a fully paid up member of the EU, including all the rights and obligations that go along with membership.
After the extension has been signed off this week, Brussels will watch, arms folded from the sidelines as the next moves are decided in Westminster.
The SNP and Lib Dems have broken with the Labour position on Brexit to push for an election on 9 December.
Their bill would tweak the 2011 Fixed-term Parliaments Act - the law which sets the time-frame for elections.
If passed, it would enable an election to take place with only a majority of one, rather than two-thirds of MPs.
It would also set the election date in stone and give PM no "wriggle room" to alter the date after MPs had voted, which he could theoretically do under the current legislation.
Lib Dem leader Jo Swinson said while her preference was for another referendum, there was not enough support in Parliament for this as things stood.
A general election, she told MPs, was the only way to stop Mr Johnson pushing through his Brexit deal in the coming weeks "on the back of Labour votes".
She said she would not insist for votes for 16-year olds as the price of her support. "The worst thing we can do for 16 and 17 year olds is to crash out without a deal"," she said.
"Leaving EU is the thing that will wreck their future."
The leader of The Independent Group for Change, former Conservative MP Anna Soubry, sent an email to her party's supporters accusing the SNP and Lib Dems of "turning their backs" on the People's Vote campaign.
The Independent Group for Change has five MPs.
Plaid Cymru, which has four MPs, said another referendum, rather than an election, was the "clearest way to end the Brexit chaos".